46 ^- MITSUKUKI; 



now continuous by its whole height with the mesoblast. 

 Its connection with the epiblast above, and with the 

 liypobhist below, remains the same as before. 



h is two sections behind g, and is tlie first section behind the 

 closure of the ventral opening of the neurenteric canal. 

 The canal is also just beginning to close above. This 

 takes place a little below the surface of the epiblast, so 

 that the latter layer is marked by a groove in the median 

 line. 



i is three sections behind /;. It is the last section in which 

 the posterior wall of the neurenteric canal is visible in 

 section. The median superficial groove, i.e., the primitive 

 groove, is well marked, and on each side of it the epiblast 

 dips down to become continuous with the mass below. 

 It may be pointed out here that the height of the sections, 

 I.e., the distance between their dorsal and ventral surfaces 

 in the median region, has been increasing from c back- 

 wards and is orreatest in h and i. A reference to the 

 dorsal and ventral surface- views shows that there is a 

 considerable thick mass, the " Endwulst," around the 

 neurenteric canal. The heis^lit decreases a";ain from this 

 point. 



j is five sections behind i. The Avell-known characteristics 

 of the vertebrate primitive streak are very distinct in this 

 section. We must consider this as about the end of the 

 " Endwulst." 



k is four sections behind /, and shows all the points of a 

 typical section through the vertebrate primitive streak. 

 The height is considerably less than in j, i, or /(. The 

 groove is deeper. 



1 is four sections behind k. The primitive groove has 

 flattened out. There is now^ only a trough-like depres- 

 sion over the epiblast, with the deepest point in the 

 median line. Over this trough and without any relation 

 to any adjacent structure there is an irregular granular 



